path: root/doc/rfc/rfc3972.txt

Network Working Group                                            T. Aura
Request for Comments: 3972                            Microsoft Research
Category: Standards Track                                     March 2005


              Cryptographically Generated Addresses (CGA)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This document describes a method for binding a public signature key
   to an IPv6 address in the Secure Neighbor Discovery (SEND) protocol.
   Cryptographically Generated Addresses (CGA) are IPv6 addresses for
   which the interface identifier is generated by computing a
   cryptographic one-way hash function from a public key and auxiliary
   parameters.  The binding between the public key and the address can
   be verified by re-computing the hash value and by comparing the hash
   with the interface identifier.  Messages sent from an IPv6 address
   can be protected by attaching the public key and auxiliary parameters
   and by signing the message with the corresponding private key.  The
   protection works without a certification authority or any security
   infrastructure.


















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  CGA Format . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  CGA Parameters and Hash Values . . . . . . . . . . . . . . . .  5
   4.  CGA Generation . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  CGA Verification . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  CGA Signatures . . . . . . . . . . . . . . . . . . . . . . . . 10
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
       7.1.  Security Goals and Limitations . . . . . . . . . . . . . 12
       7.2.  Hash Extension . . . . . . . . . . . . . . . . . . . . . 13
       7.3.  Privacy Considerations . . . . . . . . . . . . . . . . . 15
       7.4.  Related Protocols  . . . . . . . . . . . . . . . . . . . 15
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
       9.1.  Normative References . . . . . . . . . . . . . . . . . . 17
       9.2.  Informative References . . . . . . . . . . . . . . . . . 18
   Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
       A.  Example of CGA Generation. . . . . . . . . . . . . . . . . 20
       B.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . 21
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 21
   Full Copyright Statements. . . . . . . . . . . . . . . . . . . . . 22

1.  Introduction

   This document specifies a method for securely associating a
   cryptographic public key with an IPv6 address in the Secure Neighbor
   Discovery (SEND) protocol [RFC3971].  The basic idea is to generate
   the interface identifier (i.e., the rightmost 64 bits) of the IPv6
   address by computing a cryptographic hash of the public key.  The
   resulting IPv6 address is called a cryptographically generated
   address (CGA).  The corresponding private key can then be used to
   sign messages sent from the address.  An introduction to CGAs and
   their application to SEND can be found in [Aura03] and [AAKMNR02].

   This document specifies:

   o  how to generate a CGA from the cryptographic hash of a public key
      and auxiliary parameters,

   o  how to verify the association between the public key and the CGA,
      and

   o  how to sign a message sent from the CGA, and how to verify the
      signature.






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   To verify the association between the address and the public key, the
   verifier needs to know the address itself, the public key, and the
   values of the auxiliary parameters.  The verifier can then go on to
   verify messages signed by the owner of the public key (i.e., the
   address owner).  No additional security infrastructure, such as a
   public key infrastructure (PKI), certification authorities, or other
   trusted servers, is needed.

   Note that because CGAs themselves are not certified, an attacker can
   create a new CGA from any subnet prefix and its own (or anyone
   else's) public key.  However, the attacker cannot take a CGA created
   by someone else and send signed messages that appear to come from the
   owner of that address.

   The address format and the CGA parameter format are defined in
   Sections 2 and 3.  Detailed algorithms for generating addresses and
   for verifying them are given in Sections 4 and 5, respectively.
   Section 6 defines the procedures for generating and verifying CGA
   signatures.  The security considerations in Section 7 include
   limitations of CGA-based security, the reasoning behind the hash
   extension technique that enables effective hash lengths above the
   64-bit limit of the interface identifier, the implications of CGAs on
   privacy, and protection against related-protocol attacks.

   In this document, the key words MUST, MUST NOT, REQUIRED, SHALL,
   SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL are to
   be interpreted as described in [RFC2119].

2.  CGA Format

   When talking about addresses, this document refers to IPv6 addresses
   in which the leftmost 64 bits of a 128-bit address form the subnet
   prefix and the rightmost 64 bits of the address form the interface
   identifier [RFC3513].  We number the bits of the interface identifier
   starting from bit zero on the left.

   A cryptographically generated address (CGA) has a security parameter
   (Sec) that determines its strength against brute-force attacks.  The
   security parameter is a three-bit unsigned integer, and it is encoded
   in the three leftmost bits (i.e., bits 0 - 2) of the interface
   identifier.  This can be written as follows:

      Sec = (interface identifier & 0xe000000000000000) >> 61








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   The CGA is associated with a set of parameters that consist of a
   public key and auxiliary parameters.  Two hash values Hash1 (64 bits)
   and Hash2 (112 bits) are computed from the parameters.  The formats
   of the public key and auxiliary parameters, and the way to compute
   the hash values, are defined in Section 3.

   A cryptographically generated address is defined as an IPv6 address
   that satisfies the following two conditions:

   o  The first hash value, Hash1, equals the interface identifier of
      the address.  Bits 0, 1, 2, 6, and 7 (i.e., the bits that encode
      the security parameter Sec and the "u" and "g" bits from the
      standard IPv6 address architecture format of interface identifiers
      [RFC3513]) are ignored in the comparison.

   o  The 16*Sec leftmost bits of the second hash value, Hash2, are
      zero.

   The above definition can be stated in terms of the following two bit
   masks:

      Mask1 (64 bits)  = 0x1cffffffffffffff

      Mask2 (112 bits) = 0x0000000000000000000000000000  if Sec=0,
                         0xffff000000000000000000000000  if Sec=1,
                         0xffffffff00000000000000000000  if Sec=2,
                         0xffffffffffff0000000000000000  if Sec=3,
                         0xffffffffffffffff000000000000  if Sec=4,
                         0xffffffffffffffffffff00000000  if Sec=5,
                         0xffffffffffffffffffffffff0000  if Sec=6, and
                         0xffffffffffffffffffffffffffff  if Sec=7

   A cryptographically generated address is an IPv6 address for which
   the following two equations hold:

      Hash1 & Mask1  ==  interface identifier & Mask1
      Hash2 & Mask2  ==  0x0000000000000000000000000000














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3.  CGA Parameters and Hash Values

   Each CGA is associated with a CGA Parameters data structure, which
   has the following format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                                                               +
   |                                                               |
   +                      Modifier (16 octets)                     +
   |                                                               |
   +                                                               +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                    Subnet Prefix (8 octets)                   +
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Collision Count|                                               |
   +-+-+-+-+-+-+-+-+                                               |
   |                                                               |
   ~                  Public Key (variable length)                 ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   ~           Extension Fields (optional, variable length)        ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Modifier

      This field contains a 128-bit unsigned integer, which can be any
      value.  The modifier is used during CGA generation to implement
      the hash extension and to enhance privacy by adding randomness to
      the address.

   Subnet Prefix

      This field contains the 64-bit subnet prefix of the CGA.

   Collision Count

      This is an eight-bit unsigned integer that MUST be 0, 1, or 2.
      The collision count is incremented during CGA generation to
      recover from an address collision detected by duplicate address
      detection.



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   Public Key

      This is a variable-length field containing the public key of the
      address owner.  The public key MUST be formatted as a DER-encoded
      [ITU.X690.2002] ASN.1 structure of the type SubjectPublicKeyInfo,
      defined in the Internet X.509 certificate profile [RFC3280].  SEND
      SHOULD use an RSA public/private key pair.  When RSA is used, the
      algorithm identifier MUST be rsaEncryption, which is
      1.2.840.113549.1.1.1, and the RSA public key MUST be formatted by
      using the RSAPublicKey type as specified in Section 2.3.1 of RFC
      3279 [RFC3279].  The RSA key length SHOULD be at least 384 bits.
      Other public key types are undesirable in SEND, as they may result
      in incompatibilities between implementations.  The length of this
      field is determined by the ASN.1 encoding.

   Extension Fields

      This is an optional variable-length field that is not used in the
      current specification.  Future versions of this specification may
      use this field for additional data items that need to be included
      in the CGA Parameters data structure.  IETF standards action is
      required to specify the use of the extension fields.
      Implementations MUST ignore the value of any unrecognized
      extension fields.

   The two hash values MUST be computed as follows.  The SHA-1 hash
   algorithm [FIPS.180-1.1995] is applied to the CGA Parameters.  When
   Hash1 is computed, the input to the SHA-1 algorithm is the CGA
   Parameters data structure.  The 64-bit Hash1 is obtained by taking
   the leftmost 64 bits of the 160-bit SHA-1 hash value.  When Hash2 is
   computed, the input is the same CGA Parameters data structure except
   that the subnet prefix and collision count are set to zero.  The
   112-bit Hash2 is obtained by taking the leftmost 112 bits of the
   160-bit SHA-1 hash value.  Note that the hash values are computed
   over the entire CGA Parameters data structure, including any
   unrecognized extension fields.

4.  CGA Generation

   The process of generating a new CGA takes three input values: a
   64-bit subnet prefix, the public key of the address owner as a
   DER-encoded ASN.1 structure of the type SubjectPublicKeyInfo, and the
   security parameter Sec, which is an unsigned three-bit integer.  The
   cost of generating a new CGA depends exponentially on the security
   parameter Sec, which can have values from 0 to 7.






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   A CGA and associated parameters SHOULD be generated as follows:

   1. Set the modifier to a random or pseudo-random 128-bit value.

   2. Concatenate from left to right the modifier, 9 zero octets, the
      encoded public key, and any optional extension fields.  Execute
      the SHA-1 algorithm on the concatenation.  Take the 112 leftmost
      bits of the SHA-1 hash value.  The result is Hash2.

   3. Compare the 16*Sec leftmost bits of Hash2 with zero.  If they are
      all zero (or if Sec=0), continue with step 4.  Otherwise,
      increment the modifier by one and go back to step 2.

   4. Set the 8-bit collision count to zero.

   5. Concatenate from left to right the final modifier value, the
      subnet prefix, the collision count, the encoded public key, and
      any optional extension fields.  Execute the SHA-1 algorithm on the
      concatenation.  Take the 64 leftmost bits of the SHA-1 hash value.
      The result is Hash1.

   6. Form an interface identifier from Hash1 by writing the value of
      Sec into the three leftmost bits and by setting bits 6 and 7
      (i.e., the "u" and "g" bits) to zero.

   7. Concatenate the 64-bit subnet prefix and the 64-bit interface
      identifier to form a 128-bit IPv6 address with the subnet prefix
      to the left and interface identifier to the right, as in a
      standard IPv6 address [RFC3513].

   8. Perform duplicate address detection if required, as per [RFC3971].
      If an address collision is detected, increment the collision count
      by one and go back to step 5.  However, after three collisions,
      stop and report the error.

   9. Form the CGA Parameters data structure by concatenating from left
      to right the final modifier value, the subnet prefix, the final
      collision count value, the encoded public key, and any optional
      extension fields.

   The output of the address generation algorithm is a new CGA and a CGA
   Parameters data structure.

   The initial value of the modifier in step 1 SHOULD be chosen randomly
   to make addresses generated from the same public key unlinkable,
   which enhances privacy (see Section 7.3).  The quality of the random
   number generator does not affect the strength of the binding between




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   the address and the public key.  Implementations that have no strong
   random numbers available MAY use a non-cryptographic pseudo-random
   number generator initialized with the current time of day.

   For Sec=0, the above algorithm is deterministic and relatively fast.
   Nodes that implement CGA generation MAY always use the security
   parameter value Sec=0.  If Sec=0, steps 2 - 3 of the generation
   algorithm can be skipped.

   For Sec values greater than zero, the above algorithm is not
   guaranteed to terminate after a certain number of iterations.  The
   brute-force search in steps 2 - 3 takes O(2^(16*Sec)) iterations to
   complete.  The algorithm has been intentionally designed so that the
   generation of CGAs with high Sec values is infeasible with current
   technology.

   Implementations MAY use optimized or otherwise modified versions of
   the above algorithm for CGA generation.  However, the output of any
   modified versions MUST fulfill the following two requirements.
   First, the resulting CGA and CGA Parameters data structure MUST be
   formatted as specified in Sections 2 - 3.  Second, the CGA
   verification procedure defined in Section 5 MUST succeed when invoked
   on the output of the CGA generation algorithm.  Note that some
   optimizations involve trade-offs between privacy and the cost of
   address generation.

   One optimization is particularly important.  If the subnet prefix of
   the address changes but the address owner's public key does not, the
   old modifier value MAY be reused.  If it is reused, the algorithm
   SHOULD be started from step 4.  This optimization avoids repeating
   the expensive search for an acceptable modifier value but may, in
   some situations, make it easier for an observer to link two addresses
   to each other.

   Note that this document does not specify whether duplicate address
   detection should be performed and how the detection is done.  Step 8
   only defines what to do if some form of duplicate address detection
   is performed and an address collision is detected.

   Future versions of this specification may specify additional inputs
   to the CGA generation algorithm that are concatenated as extension
   fields to the end of the CGA Parameters data structure.  No such
   extension fields are defined in this document.








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5.  CGA Verification

   CGA verification takes an IPv6 address and a CGA Parameters data
   structure as input.  The CGA Parameters consist of the concatenated
   modifier, subnet prefix, collision count, public key, and optional
   extension fields.  The verification either succeeds or fails.

   The CGA MUST be verified with the following steps:

   1. Check that the collision count in the CGA Parameters data
      structure is 0, 1, or 2.  The CGA verification fails if the
      collision count is out of the valid range.

   2. Check that the subnet prefix in the CGA Parameters data structure
      is equal to the subnet prefix (i.e., the leftmost 64 bits) of the
      address.  The CGA verification fails if the prefix values differ.

   3. Execute the SHA-1 algorithm on the CGA Parameters data structure.
      Take the 64 leftmost bits of the SHA-1 hash value.  The result is
      Hash1.

   4. Compare Hash1 with the interface identifier (i.e., the rightmost
      64 bits) of the address.  Differences in the three leftmost bits
      and in bits 6 and 7 (i.e., the "u" and "g" bits) are ignored.  If
      the 64-bit values differ (other than in the five ignored bits),
      the CGA verification fails.

   5. Read the security parameter Sec from the three leftmost bits of
      the 64-bit interface identifier of the address.  (Sec is an
      unsigned 3-bit integer.)

   6. Concatenate from left to right the modifier, 9 zero octets, the
      public key, and any extension fields that follow the public key in
      the CGA Parameters data structure.  Execute the SHA-1 algorithm on
      the concatenation.  Take the 112 leftmost bits of the SHA-1 hash
      value.  The result is Hash2.

   7. Compare the 16*Sec leftmost bits of Hash2 with zero.  If any one
      of them is not zero, the CGA verification fails.  Otherwise, the
      verification succeeds.  (If Sec=0, the CGA verification never
      fails at this step.)

   If the verification fails at any step, the execution of the algorithm
   MUST be stopped immediately.  On the other hand, if the verification
   succeeds, the verifier knows that the public key in the CGA
   Parameters is the authentic public key of the address owner.  The





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   verifier can extract the public key by removing 25 octets from the
   beginning of the CGA Parameters and by decoding the following
   SubjectPublicKeyInfo data structure.

   Note that the values of bits 6 and 7 (the "u" and "g" bits) of the
   interface identifier are ignored during CGA verification.  In the
   SEND protocol, after the verification succeeds, the verifier SHOULD
   process all CGAs in the same way regardless of the Sec, modifier, and
   collision count values.  In particular, the verifier in the SEND
   protocol SHOULD NOT have any security policy that differentiates
   between addresses based on the value of Sec.  That way, the address
   generator is free to choose any value of Sec.

   All nodes that implement CGA verification MUST be able to process all
   security parameter values Sec = 0, 1, 2, 3, 4, 5, 6, 7.  The
   verification procedure is relatively fast and always requires at most
   two computations of the SHA-1 hash function.  If Sec=0, the
   verification never fails in steps 6 - 7 and these steps can be
   skipped.

   Nodes that implement CGA verification for SEND SHOULD be able to
   process RSA public keys that have the algorithm identifier
   rsaEncryption and, key length between 384 and 2,048 bits.
   Implementations MAY support longer keys.  Future versions of this
   specification may recommend support for longer keys.

   Implementations of CGA verification MUST ignore the value of any
   unrecognized extension fields that follow the public key in the CGA
   Parameters data structure.  However, implementations MUST include any
   such unrecognized data in the hash input when computing Hash1 in step
   3 and Hash2 in step 6 of the CGA verification algorithm.  This is
   important to ensure upward compatibility with future extensions.

6.  CGA Signatures

   This section defines the procedures for generating and verifying CGA
   signatures.  To sign a message, a node needs the CGA, the associated
   CGA Parameters data structure, the message, and the private
   cryptographic key that corresponds to the public key in the CGA
   Parameters.  The node also must have a 128-bit type tag for the
   message from the CGA Message Type name space.

   To sign a message, a node SHOULD do the following:

   o  Concatenate the 128-bit type tag (in network byte order) and the
      message with the type tag to the left and the message to the
      right.  The concatenation is the message to be signed in the next
      step.



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   o  Generate the RSA signature by using the RSASSA-PKCS1-v1_5
      [RFC3447] signature algorithm with the SHA-1 hash algorithm.  The
      private key and the concatenation created above are the inputs to
      the generation operation.

   The SEND protocol specification [RFC3971] defines several messages
   that contain a signature in the Signature Option.  The SEND protocol
   specification also defines a type tag from the CGA Message Type name
   space.  The same type tag is used for all the SEND messages that have
   the Signature Option.  This type tag is an IANA-allocated 128 bit
   integer that has been chosen at random to prevent an accidental type
   collision with messages of other protocols that use the same public
   key but that may or may not use IANA-allocated type tags.

   The CGA, the CGA Parameters data structure, the message, and the
   signature are sent to the verifier.  The SEND protocol specification
   defines how these data items are sent in SEND protocol messages.
   Note that the 128-bit type tag is not included in the SEND protocol
   messages because the verifier knows its value implicitly from the
   ICMP message type field in the SEND message.  See the SEND
   specification [RFC3971] for precise information about how SEND
   handles the type tag.

   To verify a signature, the verifier needs the CGA, the associated CGA
   Parameters data structure, the message, and the signature.  The
   verifier also needs to have the 128-bit type tag for the message.

   To verify the signature, a node SHOULD do the following:

   o  Verify the CGA as defined in Section 5.  The inputs to the CGA
      verification are the CGA and the CGA Parameters data structure.

   o  Concatenate the 128-bit type tag and the message with the type tag
      to the left and the message to the right.  The concatenation is
      the message whose signature is to be verified in the next step.

   o  Verify the RSA signature by using the RSASSA-PKCS1-v1_5 [RFC3447]
      algorithm with the SHA-1 hash algorithm.  The inputs to the
      verification operation are the public key (i.e., the RSAPublicKey
      structure from the SubjectPublicKeyInfo structure that is a part
      of the CGA Parameters data structure), the concatenation created
      above, and the signature.

   The verifier MUST accept the signature as authentic only if both the
   CGA verification and the signature verification succeed.






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7.  Security Considerations

7.1.  Security Goals and Limitations

   The purpose of CGAs is to prevent stealing and spoofing of existing
   IPv6 addresses.  The public key of the address owner is bound
   cryptographically to the address.  The address owner can use the
   corresponding private key to assert its ownership and to sign SEND
   messages sent from the address.

   It is important to understand that an attacker can create a new
   address from an arbitrary subnet prefix and its own (or someone
   else's) public key because CGAs are not certified.  However, the
   attacker cannot impersonate somebody else's address.  This is because
   the attacker would have to find a collision of the cryptographic hash
   value Hash1.  (The property of the hash function needed here is
   called second pre-image resistance [MOV97].)

   For each valid CGA Parameters data structure, there are 4*(Sec+1)
   different CGAs that match the value.  This is because decrementing
   the Sec value in the three leftmost bits of the interface identifier
   does not invalidate the address, and the verifier ignores the values
   of the "u" and "g" bits.  In SEND, this does not have any security or
   implementation implications.

   Another limitation of CGAs is that there is no mechanism for proving
   that an address is not a CGA.  Thus, an attacker could take someone
   else's CGA and present it as a non-cryptographically generated
   address (e.g., as an RFC 3041 address [RFC3041]).  An attacker does
   not benefit from this because although SEND nodes accept both signed
   and unsigned messages from every address, they give priority to the
   information in the signed messages.

   The minimum RSA key length required for SEND is only 384 bits.  So
   short keys are vulnerable to integer-factoring attacks and cannot be
   used for strong authentication or secrecy.  On the other hand, the
   cost of factoring 384-bit keys is currently high enough to prevent
   most denial-of-service attacks.  Implementations that initially use
   short RSA keys SHOULD be prepared to switch to longer keys when
   denial-of-service attacks arising from integer factoring become a
   problem.

   The impact of a key compromise on CGAs depends on the application for
   which they are used.  In SEND, it is not a major concern.  If the
   private signature key is compromised because the SEND node has itself
   been compromised, the attacker does not need to spoof SEND messages
   from the node.  When it is discovered that a node has been
   compromised, a new signature key and a new CGA SHOULD be generated.



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   On the other hand, if the RSA key is compromised because integer-
   factoring attacks for the chosen key length have become practical,
   the key has to be replaced with a longer one, as explained above.  In
   either case, the address change effectively revokes the old public
   key.  It is not necessary to have any additional key revocation
   mechanism or to limit the lifetimes of the signature keys.

7.2.  Hash Extension

   As computers become faster, the 64 bits of the interface identifier
   will not be sufficient to prevent attackers from searching for hash
   collisions.  It helps somewhat that we include the subnet prefix of
   the address in the hash input.  This prevents the attacker from using
   a single pre-computed database to attack addresses with different
   subnet prefixes.  The attacker needs to create a separate database
   for each subnet prefix.  Link-local addresses are, however, left
   vulnerable because the same prefix is used by all IPv6 nodes.

   To prevent the CGA technology from becoming outdated as computers
   become faster, the hash technique used to generate CGAs must be
   extended somehow.  The chosen extension technique is to increase the
   cost of both address generation and brute-force attacks by the same
   parameterized factor while keeping the cost of address use and
   verification constant.  This also provides protection for link-local
   addresses.  Introduction of the hash extension is the main difference
   between this document and earlier CGA proposals [OR01][Nik01][MC02].

   To achieve the effective extension of the hash length, the input to
   the second hash function, Hash2, is modified (by changing the
   modifier value) until the leftmost 16*Sec bits of the hash value are
   zero.  This increases the cost of address generation approximately by
   a factor of 2^(16*Sec).  It also increases the cost of brute-force
   attacks by the same factor.  That is, the cost of creating a CGA
   Parameters data structure that binds the attacker's public key with
   somebody else's address is increased from O(2^59) to
   O(2^(59+16*Sec)).  The address generator may choose the security
   parameter Sec depending on its own computational capacity, the
   perceived risk of attacks, and the expected lifetime of the address.
   Currently, Sec values between 0 and 2 are sufficient for most IPv6
   nodes.  As computers become faster, higher Sec values will slowly
   become useful.

   Theoretically, if no hash extension is used (i.e., Sec=0) and a
   typical attacker is able to tap into N local networks at the same
   time, an attack against link-local addresses is N times as efficient
   as an attack against addresses of a specific network.  The effect
   could be countered by using a slightly higher Sec value for link-




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   local addresses.  When higher Sec values (such that 2^(16*Sec) > N)
   are used for all addresses, the relative advantage of attacking
   link-local addresses becomes insignificant.

   The effectiveness of the hash extension depends on the assumption
   that the computational capacities of the attacker and the address
   generator will grow at the same (potentially exponential) rate.  This
   is not necessarily true if the addresses are generated on low-end
   mobile devices, for which the main design goals are to lower cost and
   decrease size, rather than increase computing power.  But there is no
   reason for doing so.  The expensive part of the address generation
   (steps 1 - 3 of the generation algorithm) may be delegated to a more
   powerful computer.  Moreover, this work can be done in advance or
   offline, rather than in real time, when a new address is needed.

   To make it possible for mobile nodes whose subnet prefixes change
   frequently to use Sec values greater than zero, we have decided not
   to include the subnet prefix in the input of Hash2.  The result is
   weaker than it would be if the subnet prefix were included in the
   input of both hashes.  On the other hand, our scheme is at least as
   strong as using the hash extension technique without including the
   subnet prefix in either hash.  It is also at least as strong as not
   using the hash extension but including the subnet prefix.  This
   trade-off was made because mobile nodes frequently move to insecure
   networks, where they are at the risk of denial-of-service (DoS)
   attacks (for example, during the duplicate address detection
   procedure).

   In most networks, the goal of Secure Neighbor Discovery and CGA
   signatures is to prevent denial-of-service attacks.  Therefore, it is
   usually sensible to start by using a low Sec value and to replace
   addresses with stronger ones only when denial-of-service attacks
   based on brute-force search become a significant problem.  If CGAs
   were used as a part of a strong authentication or secrecy mechanism,
   it might be necessary to start with higher Sec values.

   The collision count value is used to modify the input to Hash1 if
   there is an address collision.  It is important not to allow
   collision count values higher than 2.  First, it is extremely
   unlikely that three collisions would occur and the reason is certain
   to be either a configuration or implementation error or a denial-of-
   service attack.  (When the SEND protocol is used, deliberate
   collisions caused by a DoS attacker are detected and ignored.)
   Second, an attacker doing a brute-force search to match a given CGA
   can try all different values of a collision count without repeating
   the brute-force search for the modifier value.  Thus, if higher
   values are allowed for the collision count, the hash extension
   technique becomes less effective in preventing brute force attacks.



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7.3.  Privacy Considerations

   CGAs can give the same level of pseudonymity as the IPv6 address
   privacy extensions defined in RFC 3041 [RFC3041].  An IP host can
   generate multiple pseudo-random CGAs by executing the CGA generation
   algorithm of Section 4 multiple times and by using a different random
   or pseudo-random initial value for the modifier every time.  The host
   should change its address periodically as in [RFC3041].  When privacy
   protection is needed, the (pseudo)random number generator used in
   address generation SHOULD be strong enough to produce unpredictable
   and unlinkable values.  Advice on random number generation can be
   found in [RFC1750].

   There are two apparent limitations to this privacy protection.
   However, as will be explained below, neither is very serious.

   First, the high cost of address generation may prevent hosts that use
   a high Sec value from changing their address frequently.  This
   problem is mitigated because the expensive part of the address
   generation may be done in advance or offline, as explained in the
   previous section.  It should also be noted that the nodes that
   benefit most from high Sec values (e.g., DNS servers, routers, and
   data servers) usually do not require pseudonymity, and the nodes that
   have high privacy requirements (e.g., client PCs and mobile hosts)
   are unlikely targets for expensive brute-force DoS attacks and can
   make do with lower Sec values.

   Second, the public key of the address owner is revealed in the signed
   SEND messages.  This means that if the address owner wants to be
   pseudonymous toward the nodes in the local links that it accesses, it
   should generate not only a new address but also a new public key.
   With typical local-link technologies, however, a node's link-layer
   address is a unique identifier for the node.  As long as the node
   keeps using the same link-layer address, it makes little sense to
   change the public key for privacy reasons.

7.4.  Related Protocols

   Although this document defines CGAs only for the purposes of Secure
   Neighbor Discovery, other protocols could be defined elsewhere that
   use the same addresses and public keys.  This raises the possibility
   of related-protocol attacks in which a signed message from one
   protocol is replayed in another protocol.  This means that other
   protocols (perhaps even those designed without an intimate knowledge
   of SEND) could endanger the security of SEND.  What makes this threat
   even more significant is that the attacker could create a CGA from
   someone else's public key and then replay signed messages from a
   protocol that has nothing to do with CGAs or IP addresses.



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   To prevent the related-protocol attacks, a type tag is prepended to
   every message before it is signed.  The type tags are 128-bit
   randomly chosen values, which prevents accidental type collisions
   with even poorly designed protocols that do not use any type tags.
   Moreover, the SEND protocol includes the sender's CGA address in all
   signed messages.  This makes it even more difficult for an attacker
   to take signed messages from some other context and to replay them as
   SEND messages.

   Finally, a strong cautionary note has to be made about using CGA
   signatures for purposes other than SEND.  First, the other protocols
   MUST include a type tag and the sender address in all signed messages
   in the same way that SEND does.  Each protocol MUST define its own
   type tag values as explained in Section 8.  Moreover, because of the
   possibility of related-protocol attacks, the public key MUST be used
   only for signing, and it MUST NOT be used for encryption.  Second,
   the minimum RSA key length of 384 bits may be too short for many
   applications and the impact of key compromise on the particular
   protocol must be evaluated.  Third, CGA-based authorization is
   particularly suitable for securing neighbor discovery [RFC2461] and
   duplicate address detection [RFC2462] because these are network-layer
   signaling protocols for which IPv6 addresses are natural endpoint
   identifiers.  In any protocol that uses other identifiers, such as
   DNS names, CGA signatures alone are not a sufficient security
   mechanism.  There must also be a secure way of mapping the other
   identifiers to IPv6 addresses.  If the goal is not to verify claims
   about IPv6 addresses, CGA signatures are probably not the right
   solution.

8.  IANA Considerations

   This document defines a new CGA Message Type name space for use as
   type tags in messages that may be signed by using CGA signatures.
   The values in this name space are 128-bit unsigned integers.  Values
   in this name space are allocated on a First Come First Served basis
   [RFC2434].  IANA assigns new 128-bit values directly without a
   review.

   The requester SHOULD generate the new values with a strong random-
   number generator.  Continuous ranges of at most 256 values can be
   requested provided that the 120 most significant bits of the values
   have been generated with a strong random-number generator.

   IANA does not generate random values for the requester.  IANA
   allocates requested values without verifying the way in which they
   have been generated.  The name space is essentially unlimited, and
   any number of individual values and ranges of at most 256 values can
   be allocated.



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   CGA Message Type values for private use MAY be generated with a
   strong random-number generator without IANA allocation.

   This document does not define any new values in any name space.

9.  References

9.1.  Normative References

   [RFC3971]         Arkko, J., Ed., Kempf, J., Sommerfeld, B., Zill,
                     B., and P. Nikander, "SEcure Neighbor Discovery
                     (SEND)", RFC 3971, March 2005.

   [RFC3279]         Bassham, L., Polk, W., and R. Housley, "Algorithms
                     and Identifiers for the Internet X.509 Public Key
                     Infrastructure Certificate and Certificate
                     Revocation List (CRL) Profile", RFC 3279, April
                     2002.

   [RFC2119]         Bradner, S., "Key words for use in RFCs to Indicate
                     Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3513]         Hinden, R. and S. Deering, "Internet Protocol
                     Version 6 (IPv6) Addressing Architecture", RFC
                     3513, April 2003.

   [RFC3280]         Housley, R., Polk, W., Ford, W., and D. Solo,
                     "Internet X.509 Public Key Infrastructure
                     Certificate and Certificate Revocation List (CRL)
                     Profile", RFC 3280, April 2002.

   [ITU.X690.2002]   International Telecommunications Union,
                     "Information Technology - ASN.1 encoding rules:
                     Specification of Basic Encoding Rules (BER),
                     Canonical Encoding Rules (CER) and Distinguished
                     Encoding Rules (DER)", ITU-T Recommendation X.690,
                     July 2002.

   [RFC3447]         Jonsson, J. and B. Kaliski, "Public-Key
                     Cryptography Standards (PKCS) #1: RSA Cryptography
                     Specifications Version 2.1", RFC 3447, February
                     2003.

   [RFC2434]         Narten, T. and H. Alvestrand, "Guidelines for
                     Writing an IANA Considerations Section in RFCs",
                     BCP 26, RFC 2434, October 1998.





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   [FIPS.180-1.1995] National Institute of Standards and Technology,
                     "Secure Hash Standard", Federal Information
                     Processing Standards Publication FIPS PUB 180-1,
                     April 1995,
                     <http://www.itl.nist.gov/fipspubs/fip180-1.htm>.

9.2.  Informative References

   [AAKMNR02]        Arkko, J., Aura, T., Kempf, J., Mantyla, V.,
                     Nikander, P., and M. Roe, "Securing IPv6 neighbor
                     discovery and router discovery", ACM Workshop on
                     Wireless Security (WiSe 2002), Atlanta, GA USA ,
                     September 2002.

   [Aura03]          Aura, T., "Cryptographically Generated Addresses
                     (CGA)", 6th Information Security Conference
                     (ISC'03), Bristol, UK, October 2003.

   [RFC1750]         Eastlake, D., Crocker, S., and J. Schiller,
                     "Randomness Recommendations for Security", RFC
                     1750, December 1994.

   [MOV97]           Menezes, A., van Oorschot, P., and S. Vanstone,
                     "Handbook of Applied Cryptography", CRC Press ,
                     1997.

   [MC02]            Montenegro, G. and C. Castelluccia, "Statistically
                     unique and cryptographically verifiable identifiers
                     and addresses", ISOC Symposium on Network and
                     Distributed System Security (NDSS 2002), San Diego,
                     CA USA , February 2002.

   [RFC3041]         Narten, T. and R. Draves, "Privacy Extensions for
                     Stateless Address Autoconfiguration in IPv6", RFC
                     3041, January 2001.

   [RFC2461]         Narten, T., Nordmark, E., and W. Simpson, "Neighbor
                     Discovery for IP Version 6 (IPv6)", RFC 2461,
                     December 1998.

   [Nik01]           Nikander, P., "A scaleable architecture for IPv6
                     address ownership", draft-nikander-addr-ownership-
                     00 (work in progress), March 2001.

   [OR01]            O'Shea, G. and M. Roe, "Child-proof authentication
                     for MIPv6 (CAM)", ACM Computer Communications
                     Review 31(2), April 2001.




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   [RFC2462]         Thomson, S. and T. Narten, "IPv6 Stateless Address
                     Autoconfiguration", RFC 2462, December 1998.

















































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Appendix A.  Example of CGA Generation

   We generate a CGA with Sec=1 from the subnet prefix fe80:: and the
   following public key:

   305c 300d 0609 2a86 4886 f70d 0101 0105 0003 4b00 3048 0241
   00c2 c2f1 3730 5454 f10b d9ce a368 44b5 30e9 211a 4b26 2b16
   467c b7df ba1f 595c 0194 f275 be5a 4d38 6f2c 3c23 8250 8773
   c786 7f9b 3b9e 63a0 9c7b c48f 7a54 ebef af02 0301 0001

   The modifier is initialized to a random value 89a8 a8b2 e858 d8b8
   f263 3f44 d2d4 ce9a.  The input to Hash2 is:

   89a8 a8b2 e858 d8b8 f263 3f44 d2d4 ce9a 0000 0000 0000 0000 00
   305c 300d 0609 2a86 4886 f70d 0101 0105 0003 4b00 3048 0241
   00c2 c2f1 3730 5454 f10b d9ce a368 44b5 30e9 211a 4b26 2b16
   467c b7df ba1f 595c 0194 f275 be5a 4d38 6f2c 3c23 8250 8773
   c786 7f9b 3b9e 63a0 9c7b c48f 7a54 ebef af02 0301 0001

   The 112 first bits of the SHA-1 hash value computed from the above
   input are Hash2=436b 9a70 dbfd dbf1 926e 6e66 29c0.  This does not
   begin with 16*Sec=16 zero bits.  Thus, we must increment the modifier
   by one and recompute the hash.  The new input to Hash2 is:

   89a8 a8b2 e858 d8b8 f263 3f44 d2d4 ce9b 0000 0000 0000 0000 00
   305c 300d 0609 2a86 4886 f70d 0101 0105 0003 4b00 3048 0241
   00c2 c2f1 3730 5454 f10b d9ce a368 44b5 30e9 211a 4b26 2b16
   467c b7df ba1f 595c 0194 f275 be5a 4d38 6f2c 3c23 8250 8773
   c786 7f9b 3b9e 63a0 9c7b c48f 7a54 ebef af02 0301 0001

   The new hash value is Hash2=0000 01ca 680b 8388 8d09 12df fcce.  The
   16 leftmost bits of Hash2 are all zero.  Thus, we found a suitable
   modifier.  (We were very lucky to find it so soon.)

   The input to Hash1 is:

   89a8 a8b2 e858 d8b8 f263 3f44 d2d4 ce9b fe80 0000 0000 0000 00
   305c 300d 0609 2a86 4886 f70d 0101 0105 0003 4b00 3048 0241
   00c2 c2f1 3730 5454 f10b d9ce a368 44b5 30e9 211a 4b26 2b16
   467c b7df ba1f 595c 0194 f275 be5a 4d38 6f2c 3c23 8250 8773
   c786 7f9b 3b9e 63a0 9c7b c48f 7a54 ebef af02 0301 0001

   The 64 first bits of the SHA-1 hash value of the above input are
   Hash1=fd4a 5bf6 ffb4 ca6c.  We form an interface identifier from this
   by writing Sec=1 into the three leftmost bits and by setting bits 6
   and 7 (the "u" and "g" bits) to zero.  The new interface identifier
   is 3c4a:5bf6:ffb4:ca6c.




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   Finally, we form the IPv6 address fe80::3c4a:5bf6:ffb4:ca6c.  This is
   the new CGA.  No address collisions were detected this time.
   (Collisions are very rare.)  The CGA Parameters data structure
   associated with the address is the same as the input to Hash1 above.

Appendix B.  Acknowledgements

   The author gratefully acknowledges the contributions of Jari Arkko,
   Francis Dupont, Pasi Eronen, Christian Huitema, James Kempf, Pekka
   Nikander, Michael Roe, Dave Thaler, and other participants of the
   SEND working group.

Author's Address

   Tuomas Aura
   Microsoft Research
   Roger Needham Building
   7 JJ Thomson Avenue
   Cambridge  CB3 0FB
   United Kingdom

   Phone: +44 1223 479708
   EMail: tuomaura@microsoft.com




























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Full Copyright Statement

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions
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Acknowledgement

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